Add Energy <=> Power conversion processes

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.08 :tada:

Comparison is base (4cbffb9) 95.55% compared to head (eb1e968) 95.64%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #196 +/- ## ========================================== + Coverage 95.55% 95.64% +0.08% ========================================== Files 40 41 +1 Lines 2608 2661 +53 Branches 331 333 +2 ========================================== + Hits 2492 2545 +53 Misses 92 92 Partials 24 24 ``` | [Impacted Files](https://app.codecov.io/gh/BEMServer/bemserver-core/pull/196?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BEMServer) | Coverage Δ | | |---|---|---| | [bemserver\_core/exceptions.py](https://app.codecov.io/gh/BEMServer/bemserver-core/pull/196?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BEMServer#diff-YmVtc2VydmVyX2NvcmUvZXhjZXB0aW9ucy5weQ==) | `100.00% <100.00%> (ø)` | | | [bemserver\_core/model/timeseries.py](https://app.codecov.io/gh/BEMServer/bemserver-core/pull/196?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BEMServer#diff-YmVtc2VydmVyX2NvcmUvbW9kZWwvdGltZXNlcmllcy5weQ==) | `99.58% <100.00%> (+0.01%)` | :arrow_up: | | [bemserver\_core/process/energy\_power.py](https://app.codecov.io/gh/BEMServer/bemserver-core/pull/196?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BEMServer#diff-YmVtc2VydmVyX2NvcmUvcHJvY2Vzcy9lbmVyZ3lfcG93ZXIucHk=) | `100.00% <100.00%> (ø)` | |

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.